TELEPHONE INTERFACE MODULE (TIM)

14.4E&M LEAD CIRCUIT THEORY OF OPERATION

14.4.1 E&M INTERFACE

This card was designed to mate with any of the five available E&M signaling formats. E&M signaling usually requires a -48V supply with positive ground and must have some type of current limiting available. Q205 and Q204 are a bipolar constant current source used for current limiting.

CR201, CR202 and CR203 are 68V, 0.5W zener diodes that act with RV202-RV208 to provide electrostatic discharge protection. These devices ground any voltage spikes caused from back-EMF associated with driving relay type E&M systems.

The basic switching for this card comes from Q202/Q203 (a complementary Darlington pair). With S201 (8-position DIP switch) the relative configuration of Q202/Q203 can be manipulated. Figure 14-11is an “equivalent switching” diagram for a TYPE III configuration.

The drive for Q202 and Q203 is from optoisola- tors U213/U214. Q206 is used only in Type V interface where an inversion is required to drive Q202.

The E-Lead current sensor consists of current limit resistor R219 and an optoisolator U212. CR205 is for reverse voltage protection for the optoisolator and LED CR204.

Figure 14-11 TYPE III INTERFACE

14.4.2 SIGNAL CONDITIONING

Q201 inverts the logic to the Schmitt buffer

U211. The buffer output is used to charge and discharge C227 through R230 and the series output resistance of CMOS gate U211. When the buffer is on, C227 is charged through R230 which sets up an exponential ramp voltage on C227. R231/R232 set a reference of 2.17V and R233/R234 set a reference at 2.86V. As C227 is charging from 0 to +5V the comparator sequentially trips causing U213 to trip first and U214 to trip ~ 0.3 ms later (see Figure 14-12).

Figure 14-12 CHARGING CHARACTERISTICOF C227

When a "0" is written to Q201 the reverse action occurs with the discharge of C227 through R230. The reason for this sequencing is to emulate a "break- before-make" switch. By sequencing the drive to the optoisolators U204 becomes an equivalent SPDT switch. When the output of the comparators goes low, they activate U213/U214.

The E-Lead output from U212 is debounced by dual retriggerable monostable multivibrator U210. It is configured as two one-shot multivibrators with RC time constants of 5 ms and 10 ms respectively. This circuit debounces transients in both E-Lead signal states (low or high).

The first one-shot multi vibrator is used to debounce a transient “1” during a logic “0”. The Q output of the first one-shot is NANDed with its input (R221/C226 are used to eliminate a possible race condition that may occur through the one-shot) in which the output of U211, pin 3 goes low only when the input pulse width exceeds 5 ms. This output is fed into U210, pin 12 which is being retriggered by the

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